1. Field of the Invention
The present disclosure relates to railroads generally, and more particularly, to methods and systems for using passive signaling in jointless track circuits.
2. Discussion of Related Art
Conventional track circuits use signaling points to monitor a block of railroad track for the presence of trains and broken rails. Signals transmitted and/or received by the signaling points indicating the block state (e.g., whether occupied, empty, or containing a broken rail) are used to directly control the wayside signal aspects, and to send information to the train (via cab signals in the rail) or a central office (via remote communication links).
Blocks of railroad track are separated from each other by insulative joints (e.g., pieces of electrically insulative material), which are interposed between sections of rail. Use of jointed tracks, however, has several disadvantages. First, the pieces of electrically insulative material are expensive to install and maintain, and tend to deteriorate over time. Additionally, the distance between signaling points is limited because leakage current flows through the ballast (e.g., the material under and/or between the rails that forms or rests on the railroad bed), thereby attenuating an applied voltage between the rails. The attenuation typically occurs exponentially with distance from the source signaling point.
The current sensed at a receiving signal point is typically compared to a threshold value, and decisions about track occupancy, broken rails, and bits (e.g., codes, or signal aspects) are made based on this threshold. Since ballast leakage can vary with time and weather conditions, the threshold must be set to accommodate these changes while meeting the detection criteria for track occupancy (a short across the rails) and broken rails (an open break in a rail). A disadvantage is that this fixed threshold represents a joint optimization for detecting track occupancy, broken rails, and communication, but is typically not optimized for any one function.
Existing approaches to jointless track circuits, used for example, in passenger rail systems, apply audio frequencies (@1 kHz to @10 kHz) voltages to the railroad track. The voltages are confined to a section of track by tuned shunts placed across the track at the block boundaries. The problem with this type of jointless track circuit is that the signaling points can be located only about 0.5 miles apart due to the low-pass filtering effect of the rail inductance. This type of circuit is not practical for rail applications requiring block lengths longer than 0.5 miles.
A solution is needed that eliminates the insulated joints previously used to define a block of railroad track; that significantly extends the distance between signaling points; and that provides an inexpensive means for sensing track conditions. Additionally, to accommodate long distances between signaling points, it would be advantageous to place sensors along the track to help determine changes in the track model (e.g., to sense track conditions), or to act as communication repeaters. Such solutions will eliminate the maintenance costs and operational downtime associated with failed insulative joints.